"In the case of an influenza pandemic, existing vaccines would likely be ineffective against a radically new strain. Developing and globally distributing a new pandemic influenza vaccine will likely take 4-6 months (4 months to produce first doses of vaccine, and 6 months to produce enough to give to a large number of people), even while mathematical models demonstrate that pandemic influenza could spread globally within 6 months. Another complicating factor to pandemic influenza vaccine production involves how the vaccine is made. Since the 1940s, seasonal and pandemic influenza vaccines have been produced in chicken eggs. The virus is introduced in the allantoic fluid of the fertilized egg (this is the fluid that bathes the embryo and yolk sac), and it replicates in the membrane surround the fluid. After about three days, the virus-containing fluid is harvested from each egg, and the rest of the manufacturing process proceeds. Dependence on egg-based vaccine production is, however, problematic even with non-pandemic seasonal influenza vaccine. First, eggs must be available in large quantities when vaccine production is to begin. Any disruption in egg supply – such as disease affecting chickens, or bad weather interfering with the shipping of eggs – can mean a delay in vaccine production. Second, some influenza strains grow more slowly or less robustly than others, which can result in delays or in lower yields of vaccine virus from each egg. Third, it is possible that some viral vaccine strains, given the origin of some influenza viruses in birds, may be toxic to eggs. In that case, egg-based influenza vaccine production methods would be useless."